BERKASOVO, Serbia (AP) — Thousands of people trying to reach the heart of Europe surged across Serbia's border into Croatia in October, 2015 after authorities eased restrictions that had left them stranded for days in ankle-deep mud and rain.

The miserable wave of humanity left behind a field scattered with soaked blankets, mud-caked clothing and water-logged tents as they headed for Slovenia, the next obstacle to their quest to reach richer European Union nations via the Balkans.

Surprise move allowed an estimated 3,000 more migrants to enter Croatia bound for its small Alpine neighbor, which also has been struggling to slow the flow of humanity across its frontiers — and faced another wave of trekkers seeking to reach Austria and Germany to the north.

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A migrant with one leg sits on the border in a wheelchair with a friend, waiting to cross a border line between Serbia and Croatia, near the village of Berkasovo, Serbia, Monday, Oct. 19, 2015. Tension was building among thousands of migrants as they remained stranded in fog and cold weather in the Balkans on Sunday in their quest to reach a better life in Western Europe, two days after Hungary closed its border with Croatia and the flow of people was redirected to a much slower route via Slovenia. (AP Photo/Darko Vojinovic)

"Without any announcement, the borders opened. When the borders opened, everybody rushed," said Melita Sunjic, a spokeswoman for the U.N. refugee agency, who was stationed at the Serb-Croat border.

Many had discarded their mud-soaked socks and walked only in sandals or slippers through the ankle-deep muck in a driving rain, frigid winds and fog. Some who had lost limbs during the civil war in Syria were aided by friends pushing their wheelchairs down a country lane that, since Saturday, had been blocked by Croat police.

Now the officers stood aside to permit asylum-seekers by the thousands to walk toward buses for transport north — where they would become Slovenia's problem.

Croatia's prime minister, Zoran Milanovic, said his country had hoped to minimize the flow of people following Hungary's decision to seal its border with Croatia, but conditions on the poorly sheltered Serb side of the border had quickly grown unbearable.

"It's apparent that this is no solution, so we will let them through. We will send them toward Slovenia," Milanovic said.

Aid workers handed out blue rain ponchos and bags of food to travelers, many of them slipping in the mud as they walked across the border. Officials on the Croat side planned to bus the newcomers either to a Croat refugee camp or — far more likely, given asylum seekers' reluctance to stop before reaching their desired destinations — to the Slovenian border.

Slovenia's Interior Ministry said some 5,000 people had reached its borders Monday, and most were allowed to enter, with at least 900 reaching Austria by the evening. Slovenia had vowed to let in no more than 2,500 migrants per day.

Slovenian President Borut Pahor insisted his country would accept only as many travelers as could be funneled directly on to Austria. He said Slovenia was determined not to be left holding the bag should Austria or Germany suddenly stop accepting refugee applicants.

"As long as Austria will control the flow of refugees, we will have to do the same on the Slovenian-Croatian border," Pahor said.

An empty field near the Serbian border town of Berkasovo was littered with discarded belongings in an illustration of just how desperately those who had been stuck there wanted to cross into Croatia. Only hours before, its rows of tents had been packed with people fleeing war and poverty in the Middle East, Asia and Africa. Now only a few hundred remained. Dozens could be seen in the distance walking into Croatia, many carrying children on their backs.

Left behind in the scramble were stuffed toys, a milk bottle, a child's rubber boot, crayons scattered in the mud and soaked blankets. Cleaning crews could be seen collecting the scattered belongings with shovels in hopes of clearing the boggy field in time for the next migrant wave coming north from Macedonia.

One of the last to cross into Croatia on Monday was a 28-year-old Syrian who had lost a leg in that country's civil war and was being pushed by friends in a mud-caked wheelchair. The group stared, eyes vacant with exhaustion, at nearby Croat cornfields as the man, who gave only his first name, Less, lit a cigarette with shaking hands.

"We have no more money, no jacket, no food," he said, pleading to be permitted to reach Germany without further delays.

Officials in Serbia, Slovenia and Croatia all accused each other of making a bad situation worse.

Slovenia accused the Croats of breaking an agreement to limit the number of migrants crossing into its territory to 2,500 per day. Croatian officials insisted no such deal could be enforced because they lacked legal powers to confine travelers to Croat emergency shelters, which remain less than half full.

When the day's first train carrying an estimated 1,800 people stopped near Slovenia shortly after midnight, they found their path blocked in both directions by rival deployments of Croat and Slovene police, each arguing the trekkers must seek shelter in the other country.

This created a no-man's land on the border, where many were forced to spend the night in the open in the bitter cold and pelting rain. Some piled up soggy tree branches for fires.

"It's completely unacceptable," said Slovene Interior Minister Vesna Gyorkos Znidar, who accused Croatia of seeking to dump "an unlimited number of immigrants" on Slovenia rather than make an effort to them into staying at Croatian shelters.

But Croatia retorted that it, too, was being unfairly burdened by unrelenting flows from Serbia, where U.N. officials estimate another 10,000 asylum seekers — more than double the summer's typical flow — are currently traveling north to Croatia.

Before the Croat authorities lifted the border restrictions on Monday, parents desperate to get their children out of the cold and rain could be seen handing them over the security barriers to police. Many others fed up with waiting in the rain tried to outflank police positions, walking through muddy orchards and cornfields.

"We are in cold weather and the place is not good," said Farouk al-Hatib, a Syrian who was waiting to cross from Serbia. "Our message for the governments is to take into consideration our suffering."

Croatian government leaders argue that it's pointless, if not impossible, to stop people who overwhelmingly express determination to reach wealthier nations in Western Europe, chiefly Germany.

"The Republic of Croatia has asked these refugees to stay at our reception centers until their status is resolved, but they all refuse," said Matija Posavec, governor of Medjimurje, Croatia's northernmost county bordering Slovenia. "They could have stayed on board the train. They could have stayed at the reception centers, but none of them really want that. ... They just want to pass."

A group of migrants walk past a puddle after crossing from Croatia, in Brezice, Slovenia Monday, Oct. 19, 2015. Croatia's interior minister has rejected Slovenia's accusations that Croatia broke an agreement on limiting the numbers crossing their border to 2,500 a day, saying the Slovenes have kept changing the figure. (AP Photo/Darko Bandic)

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A group of migrants walk on the road near a border line between Serbia and Croatia, near the village of Berkasovo, Serbia, Monday, Oct. 19, 2015. Tension was building among thousands of migrants as they remained stranded in fog and cold weather in the Balkans on Sunday in their quest to reach a better life in Western Europe, two days after Hungary closed its border with Croatia and the flow of people was redirected to a much slower route via Slovenia. (AP Photo/Darko Vojinovic)

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Group of migrants walking on the road near a border line between Serbia and Croatia, near the village of Berkasovo, Serbia, Monday, Oct. 19, 2015. Tension was building among thousands of migrants as they remained stranded in fog and cold weather in the Balkans on Sunday in their quest to reach a better life in Western Europe, two days after Hungary closed its border with Croatia and the flow of people was redirected to a much slower route via Slovenia. (AP Photo/Darko Vojinovic)

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Migrants and refugees arrive on a dinghy from the Turkish coast to the northeastern Greek island of Lesbos, Monday, Oct. 19, 2015. More than 600,000 people, mostly Syrians, have reached Europe since the beginning of this year. (AP Photo/Santi Palacios)

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A Syrian family triy to warm up after their arrival from the Turkish coast to the northeastern Greek island of Lesbos in a dinghy, Monday, Oct. 19, 2015. More than 600,000 people, mostly Syrians, have reached Europe since the beginning of this year. (AP Photo/Santi Palacios)

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Migrants walk on a dyke after crossing from Croatia, in Brezice, Slovenia Monday, Oct. 19, 2015. Croatia's interior minister has rejected Slovenia's accusations that Croatia broke an agreement on limiting the numbers crossing their border to 2,500 a day, saying the Slovenes have kept changing the figure. (AP Photo/Darko Bandic)

A group of migrants move below a police dog after crossing from Croatia, in Brezice, Slovenia Monday, Oct. 19, 2015. Croatia's interior minister has rejected Slovenia's accusations that Croatia broke an agreement on limiting the numbers crossing their border to 2,500 a day, saying the Slovenes have kept changing the figure. (AP Photo/Darko Bandic)

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Migrants are prevented from crossing into Slovenia at a border crossing between Croatia and Slovenia, in Trnovec Croatia , Monday, Oct. 19, 2015. Hundreds of migrants have spent the night in rain and cold at Croatia's border after being refused entry into Slovenia. (AP Photo/Darko Bandic)

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Migrants reach out for bread distributed by volunteers, at a border crossing between Croatia and Slovenia, in Trnovec, Monday, Oct. 19, 2015. Hundreds of migrants have spent the night in rain and cold at Croatia's border after being refused entry into Slovenia. (AP Photo/Darko Bandic)

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Migrants reach out for bread distributed by volunteers, at a border crossing between Croatia and Slovenia, in Trnovec, Croatia Monday, Oct. 19, 2015. Hundreds of migrants have spent the night in rain and cold at Croatia's border after being refused entry into Slovenia. (AP Photo/Darko Bandic)